The present invention relates to an air-fuel ratio feedback control apparatus and method of an internal combustion engine and especially to the technology for feedback controlling to a target air-fuel ratio an air-fuel ratio of a combustion mixture using a sliding mode control.
It is common to feedback control to a target value an air-fuel ratio of a combustion mixture for the purposes of purification of exhaust gas and improvement of fuel economy in an internal combustion engine for vehicle.
For the above mentioned air-fuel ratio feedback control, it is common that an air-fuel ratio is detected by an air-fuel ratio sensor disposed in an exhaust passage, and a fuel injection quantity is feedback controlled by a proportional control action, an integral control action and a derivative control action based upon an air-fuel ratio deviation so that a detection value of the air-fuel ratio to a target air-fuel ratio.
On the other hand, a sliding mode control is well known as a control with a high robust performance suppressing an influence of disturbance. A feedback control of an air-fuel ratio using this sliding mode control is disclosed in Japanese Unexamined Patent Publication 8-232713.
With an air-fuel ratio control apparatus disclosed in the above mentioned Japanese Unexamined Patent Publication No. 8-232713, an adaptation control and an observation control are used for quickly converging a large air-fuel ratio deviation. However, the adaptation control and observation control involves complicated control designs, respectively, and require a large memory capacities. Therefore, such controls are difficult to be applied to commercial vehicles.
The present invention has been made in view of the foregoing problems, and has an object of providing an air-fuel ratio feedback control apparatus and method using a sliding mode control, capable of quickly converging a large air-fuel ratio deviation with a simple control structure.
In order to achieve the above object, with the present invention, the construction is such that a non-linear term and a linear term are computed in order to approach a detection value of an air-fuel ratio of a combustion mixture to a target air-fuel ratio based upon the detection value of the air-fuel ratio of the combustion mixture and the target air-fuel ratio, the non-linear term and the inert term are added to be output as an air-fuel ratio feedback correction coefficient for correcting a fuel injection quantity, and a gain to be used for computing the linear term is set based upon a deviation between the detection value of the air-fuel ratio and the target air-fuel ratio.
According to this construction, the linear term is computed based upon the gain corresponding to the deviation between the detection value of the air-fuel ratio and the target air-fuel ratio. An air-fuel ratio feedback correction coefficient is computed from the linear term and the non-linear term computed separately. Then, the air-fuel ratio of the combustion mixture is corrected by correcting the fuel injection quantity with the air-fuel ratio feedback correction coefficient.
The gain to be used for computing the linear term may become greater, as an absolute value of the deviation between the detection value of the air-fuel ratio and the target air-fuel ratio becomes greater.
As described above, a correction amount by the linear term become s gore ter as the deviation becomes greater, by making the gain to be used of r compu ting the linear term greater as the deviation of an actual air-fuel ratio to the target air-fuel ratio becomes greater.
The non-linear term may be computed as follows;
UNL=GNLxc3x97(air-fuel ratio detection valuexe2x88x92target air-fuel ratio)/(|air-fuel ratio detection valuexe2x88x92target air-fuel ratiol)+UNL(OLD),
wherein the non-linear term is UNL, a previous value of the non-linear term is UNL (OLD) and a gain is GNL.
According to the above equation, a switching line (S=0) is set as S=air-fuel ratio detection valuexe2x88x92target air-fuel ratio, and the positive/negative of the gain GNL is switched whenever the air-fuel ratio crosses the switching line to be added to the non-linear term UNL up to the previous time.
Here a gain correction value for correcting the gain in the computation of the non-linear term may be computed in accordance with an engine intake air quantity.
The gain in the computation of the non-linear term is corrected in response to a change in a detection delay time of air-fuel ratio due to the intake air quantity. Since the delay time becomes longer as the intake air quantity is less, the gain to be used for computation of the non-linear term is made smaller as the intake air quantity is smaller, thereby avoiding overshoot.
The linear term may be computed as follows;
ULxe2x88x92GLxc3x97(air-fuel ratio detection valuexe2x88x92target air-fuel ratio)/air-fuel ratio detection value
wherein the linear term is UL is and a gain is GL.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.